All life depends on photosynthetic carbon fixation in which CO2 is converted to organic compounds in the presence of water and light. However, this is an inefficient process, particularly in C3 plants, because of a competing process called photorespiration. Photorespiration results in the release of about a quarter of the carbon that is fixed by photosynthesis. The inefficiency of C3 photosynthesis is largely due to the enzyme ribulose-1,5-bisphosphate carboxylase oxygenase (Rubisco) that catalyzes two competing reactions, carboxylation and oxygenation. Carboxylation leads to net fixed carbon dioxide and oxygenation utilizes oxygen and results in a net loss of carbon. The relative concentrations of carbon dioxide and oxygen and the temperature as well as water availability determine which reaction occurs or dominates. Thus, C3 plants do not grow efficiently in hot and/or dry areas because, as the temperature increases, Rubisco incorporates more oxygen. Some plants, such as C4 and CAM (Crassulacean acid metabolism) plants, have developed mechanisms that reduce the effect of photorespiration by more efficiently delivering carbon dioxide to Rubisco, thereby outcompeting the oxygenase activity.